1. Field of the Invention
The present invention relates generally to sounding radars and, more particularly, to methods for suppressing cross-track clutter by distinguishing between “signal” and “clutter” based on differences in their polarimetric signatures.
2. Description of the Related Art
Sounding by a down-looking radar from a large stand-off range, as from a high-altitude aircraft or an orbiting spacecraft measures reflectivity as a function of penetration into a medium such as ice sheets or dry soil. Sounding radars generally use long wavelengths (often six meters or more), since penetration depth increases in proportion to wavelength. As a consequence, the width of the antenna pattern tends to be large in both the along-track direction and the cross-track direction.
The intended direction of measurement is at nadir (directly below the radar), but the wide antenna pattern illuminates large areas of terrain from which strong reflections may arrive at the radar at the same time as the intended reflections (desired signals) from layers internal to the sounding medium. These desired reflections (signals) are relatively weak, and may be obscured by the off-nadir returns. The latter are known as clutter.
Offending clutter returns in the along-track direction may be suppressed or avoided by partially-coherent Doppler processing. However, the clutter that arises from off-nadir reflections in the cross-track direction remain problematic and in many applications becomes the dominant factor limiting radar sounding performance.
As is well known, right-circular “R” and left-circular “L” fields are orthogonally polarized with respect to each other. In response to illumination by a circularly polarized EM field, the dominant sense of received circular polarization is opposite to the transmitted sense. (Thus the paradox: for circularly polarized radars the “like-polarized” and “cross-polarized” concepts are reversed relative to the more familiar case for linear polarizations.)
Transmitting R usually results in L-polarized backscatter being stronger, so that R becomes the cross-polarized receive state. This is because odd-bounce reflection usually dominates, as from specular surfaces, Bragg scattering from a distributed scene, or trihedrals (3-sided corners, either natural or fabricated).
In contrast, double-bounce backscatter, such as from dihedral reflectors, imposes an even number of phase reversals in the linear EM component that is aligned with the dihedral's axis, in which case stronger backscatter is observed in the same-sense circular polarity. Double-bounce reflections of circularly-polarized waves are indicated rather sensitively through their corresponding Stokes parameters, specifically, their relative phase. In the case of a lossless dihedral, the phase would differ by 180° relative to that from a single-bounce scattering surface or from alternative odd-bounce shapes.
Sounding radars need to be designed to take advantage of the different polarization characteristics of “signal” and “clutter”. The fundamental property to be exploited is that backscatter from (layers at) depth is single-bounce, whereas off-nadir clutter is usually dominated by double-bounce reflections. A generalized version of this property is that the desired depth signals retain polarization characteristics that differ from those of clutter. What is needed then are polarimetric clutter suppression methods that depend on the extent of polarization differences, and the ability to predict them (or to recognize and adapt to them through dynamic processing algorithms).